It is generally accepted that 1.alpha.,25-dihydroxycholecalciferol (1.alpha.,25-dihydroxyvitamin D.sub.3), the rapid-acting, natural metabolite of vitamin D.sub.3, is more active than vitamin D.sub.3 for intestinal calcium transport and bone calcium mobilization. This subject is extensively reviewed by DeLuca, et al. in Physiological Reviews, 53, 327 (1973).
In 1972 DeLuca, et al. described a 21-step synthesis of non-crystalline 1.alpha.,25-dihydroxycholecalciferol starting from i-homocholanic acid in Tetrahedron Letters, 4147 (1972). In 1974 Barton et al. described an 8-step synthesis of crystalline 1.alpha.,25-dihydroxycholecalciferol starting from 1.alpha.,25-diacetoxycholesterol 3-acetate, irradiating 1.alpha.,25-diacetoxy-7-dehydrocholesterol 3-acetate to a mixture of its photoisomers, equilibrating the mixture of photoisomers and isolating the desired 1.alpha.,25-dihyroxyvitamin D.sub.3 in J. C. S. Chem. Comm., 203 (1974).
Recently Iacobelli, Narwid and Uskokovic described a new synthesis of 1.alpha.,25-dihydroxycholesterol and the corresponding 1,3,25-triacetate in U.S. Patent Application Ser. No. 415,186. The utilization of these cholesterol derivatives in a process for the preparation of 1.alpha.,25-dihydroxyvitamin D.sub.3, overcoming the inherent difficulties of the two previously reported syntheses and thereby making this important metabolite of vitamin D.sub.3 readily available for pharmacological, clinical and therapeutic use would represent a major advance in the vitamin D field.